path: root/guestfs.pod

=encoding utf8

=head1 NAME

guestfs - Library for accessing and modifying virtual machine images

=head1 SYNOPSIS

 guestfs_h handle = guestfs_create ();
 guestfs_add_drive (handle, "guest.img");
 guestfs_launch (handle);
 guestfs_wait_ready (handle);
 guestfs_mount (handle, "/dev/sda1", "/");
 guestfs_touch_file (handle, "/hello");
 guestfs_sync (handle);
 guestfs_close (handle);

=head1 DESCRIPTION

Libguestfs is a library for accessing and modifying guest disk images.
Amongst the things this is good for: making batch configuration
changes to guests, getting disk used/free statistics (see also:
virt-df), migrating between virtualization systems (see also:
virt-p2v), performing partial backups, performing partial guest
clones, cloning guests and changing registry/UUID/hostname info, and
much else besides.

Libguestfs uses Linux kernel and qemu code, and can access any type of
guest filesystem that Linux and qemu can, including but not limited
to: ext2/3/4, btrfs, FAT and NTFS, LVM, many different disk partition
schemes, qcow, qcow2, vmdk.

Libguestfs provides ways to enumerate guest storage (eg. partitions,
LVs, what filesystem is in each LV, etc.).  It can also run commands
in the context of the guest.  Also you can mount guest filesystems on
the host (requires root privs and NFS).

Libguestfs is a library that can be linked with C and C++ management
programs (or management programs written in other languages, if people
contribute the language bindings).  You can also use it from shell
scripts or the command line.

=head1 CONNECTION MANAGEMENT






=head1 CONFIGURATION MANAGEMENT






=head1 HIGH-LEVEL API









=head1 ERROR HANDLING






=head1 STATE MACHINE AND LOW-LEVEL EVENT API

Internally, libguestfs is implemented by running a virtual machine
using L<qemu(1)>.  QEmu runs as a child process of the main program,
and most of this discussion won't make sense unless you understand
that the complexity is dealing with the (asynchronous) actions of the
child process.

=head2 STATE MACHINE

libguestfs uses a state machine to model the child process:

                         |
                    guestfs_create
                         |
                         |
                     ____V_____
                    /          \
                    |  CONFIG  |
                    \__________/
                     ^ ^   ^  \
                    /  |    \  \ guestfs_launch
                   /   |    _\__V______
                  /    |   /           \
                 /     |   | LAUNCHING |
                /      |   \___________/
               /       |       /
              /        |  guestfs_wait_ready
             /         |     /
    ______  /        __|____V
   /      \ ------> /        \
   | BUSY |         | READY  |
   \______/ <------ \________/

The normal transitions are (1) CONFIG (when the handle is created, but
there is no child process), (2) LAUNCHING (when the child process is
booting up), (3) alternating between READY and BUSY as commands are
issued to, and carried out by, the child process.

The guest may be killed by C<guestfs_kill_subprocess>, or may die
asynchronously at any time (eg. due to some internal error), and that
causes the state to transition back to CONFIG.

Configuration commands for qemu such as C<guestfs_add_drive> can only
be issued when in the CONFIG state.

The high-level API offers two calls that go from CONFIG through
LAUNCHING to READY.  C<guestfs_launch> is a non-blocking call that
starts up the child process, immediately moving from CONFIG to
LAUNCHING.  C<guestfs_wait_ready> blocks until the child process is
READY to accept commands (or until some failure or timeout).  The
low-level event API described below provides a non-blocking way to
replace C<guestfs_wait_ready>.

High-level API actions such as C<guestfs_mount> can only be issued
when in the READY state.  These high-level API calls block waiting for
the command to be carried out (ie. the state to transition to BUSY and
then back to READY).  But using the low-level event API, you get
non-blocking versions.  (But you can still only carry out one
operation per handle at a time - that is a limitation of the
communications protocol we use).

Finally, the child process sends asynchronous messages back to the
main program, such as kernel log messages.  Mostly these are ignored
by the high-level API, but using the low-level event API you can
register to receive these messages.

=head2 SETTING CALLBACKS TO HANDLE EVENTS

The child process generates events in some situations.  Current events
include: receiving a reply message after some action, receiving a log
message, the child process exits, &c.

Use the C<guestfs_set_*_callback> functions to set a callback for
different types of events.

Only I<one callback of each type> can be registered for each handle.
Calling C<guestfs_set_*_callback> again overwrites the previous
callback of that type.  Cancel all callbacks of this type by calling
this function with C<cb> set to C<NULL>.

=head2 NON-BLOCKING ACTIONS

C<guestfs_set_reply_callback> is the most interesting callback to
play with, since it allows you to perform actions without blocking.

For example:

 do_it ()
 {
   start_call ();
   guestfs_main_loop_run (); /* --> blocks, then calls my_cb */
 }

 start_call ()
 {
   guestfs_set_reply_callback (handle, my_cb, data);
   guestfs_nb_[action] (handle, [other parameters ...]);
   /* returns immediately */
 }
 
 my_cb (guestfs_h handle, void *data)
 {
   retval = guestfs_nb_[action]_r (handle);
   /* ... */
 }

There are C<guestfs_nb_*> and C<guestfs_nb_*_r> functions
corresponding to (very nearly) every C<guestfs_*> action in the
high-level API.

=head2 guestfs_set_reply_callback

 void guestfs_set_reply_callback (guestfs_handle h,
                                  guestfs_reply_cb cb,
                                  void *opaque);

The callback function C<cb> will be called whenever a reply is
received from the child process.  (This corresponds to a transition
from the BUSY state to the READY state).

Note (I<important!>) that high-level API calls overwrite this
callback.

=head2 guestfs_set_log_message_callback

 void guestfs_set_log_message_callback (guestfs_handle h,
                                        guestfs_log_message_cb cb,
                                        void *opaque);

The callback function C<cb> will be called whenever qemu or the guest
writes anything to the console.

Use this function to capture kernel messages and similar.

Normally there is no log message handler, and log messages are just
discarded.

=head2 guestfs_set_subprocess_quit_callback

 void guestfs_set_subprocess_quit_callback (guestfs_handle h,
                                            guestfs_subprocess_quit_cb cb,
                                            void *opaque);

The callback function C<cb> will be called when the child process
quits, either asynchronously or if killed by
C<guestfs_kill_subprocess>.  (This corresponds to a transition from
any state to the CONFIG state).

=head2 guestfs_set_ready_callback

 void guestfs_set_ready_callback (guestfs_handle h,
                                  guestfs_ready_cb cb,
                                  void *opaque);

The callback function C<cb> will be called when the child process
becomes ready.  (This corresponds to a transition from I<either>
LAUNCHING I<or> BUSY to the READY state).

You can use this instead of C<guestfs_wait_ready> to implement a
non-blocking wait for the child process to finish booting up.

=head2 EVENT MAIN LOOP

To use the low-level event API, you have to provide an event "main
loop".  You can write your own, but if you don't want to write one,
two are provided for you:

=over 4

=item libguestfs-poll

A simple main loop that is implemented using L<poll(2)>.

This is the default main loop unless you call C<guestfs_set_main_loop>
or C<guestfs_glib_set_main_loop>.

=item libguestfs-glib

An implementation which can be used with GLib and GTK+ programs.  You
can use this to write graphical (GTK+) programs which use libguestfs
without hanging during long or slow operations.

=back

=head2 guestfs_set_main_loop

 void guestfs_set_main_loop (guestfs_main_loop *);

This call sets the current main loop to the list of callbacks
contained in the C<guestfs_main_loop> structure.

Only one main loop implementation can be used by libguestfs, so
calling this replaces the previous one.  (So this is something that
has to be done by the main program, but only the main program "knows"
that it is a GTK+ program or whatever).

You should call this early in the main program, certainly before
calling C<guestfs_create>.

=head2 guestfs_glib_set_main_loop

 void guestfs_glib_set_main_loop (GMainLoop *);

This helper calls C<guestfs_set_main_loop> with the correct callbacks
for integrating with the GLib main loop.

The libguestfs-glib main loop is contained in a separate library, so
that libguestfs doesn't depend on the whole of GLib:

 #include <glib.h>
 #include <guestfs-glib.h>

 main ()
 {
   GMainLoop *loop =
     g_main_loop_new (g_main_context_default (), 1);
   ...
   guestfs_glib_set_main_loop (loop);
   ...
   g_main_loop_run (loop);
 }

To use this main loop you must link with C<-lguestfs-glib>.  (See also
the GLib and GTK+ documentation).

=head2 guestfs_main_loop_run

 void guestfs_main_loop_run (void);

This calls the main loop.

For some types of main loop you may want or prefer to call another
function, eg. C<g_main_loop_run>, or the main loop may already be
invoked by another part of your program.  In those cases, ignore this
call.

=head2 guestfs_main_loop_quit

 void guestfs_main_loop_quit (void);

This instructs the main loop to quit.  In other words,
C<guestfs_main_loop_run> will return.

For some types of main loop you may want or prefer to call another
function, eg. C<g_main_loop_quit>.  In those cases, ignore this call.

=head2 WRITING A CUSTOM MAIN LOOP

This isn't documented.  Please see the libguestfs-poll and libguestfs-glib
implementations.

=head1 SEE ALSO

L<qemu(1)>





=head1 AUTHORS

Richard W.M. Jones (C<rjones at redhat dot com>)

=head1 COPYRIGHT

Copyright (C) 2009 Red Hat Inc.
L<http://et.redhat.com/~rjones/libguestfs>

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA